Pelletizing is a process for producing a uniform particle size of newly produced or recycled plastic resins. The petroleum industry uses this process to produce pelletized polyethylene, polypropylene, and other polymeric materials with filler materials in them to allow more efficient handling and processing of the materials. The pelletizing process begins with molten polymer from an extruder being forced through a die to form multiple strands of polymer resin. Typically, the pelletizing process is performed under water where the strands are cut by a rotating knife passing along the surface of the die face immediately upon exiting the die. This operation takes place in a closed environment as water circulates through to both cool the die face and to carry the pellets out of the closed environment. The pellets are then transferred to a dewatering/drying system prior to final packing or further processing.
Generally the die face of a pelletizer is formed of a different material than the body of the pelletizer or may be coated with a different material. Because most of the wear on the pelletizer occurs at the face, the use of a hard, wear resistant, and corrosion resistant die face material allows for longer life of the pelletizer. The die face material may be replaced several times before the die body must be changed. Die face materials are subject to a range of deleterious environmental conditions such as, for example, temperature extremes, submersion in a water environment, and constant surface abrasion from the flowing polymer material and movement of the cutting knives causing cavitation. In addition to being hard and wear resistant, a die face material also should have low thermal conductivity and high corrosion/cavitation resistance.
The two most common die face materials used as wear pads and orifice nibs today are ferro-titanium carbide (Ferro-TiC) and tungsten carbide cobalt (WC—Co) alloys. The wear pads and orifice nibs are embedded in a stainless steel alloy and/or a ceramic material of the die face plate. Ferro-TiC is a machineable and hardenable alloy/steel bonded titanium carbide. Ferro-TiC is typically a metal matrix composite of titanium carbide (TiC) plus chromium (Cr), molybdenum (Mo), carbon-iron alloy (C—Fe) and/or titanium. For example, a typical Ferro-TiC composition, as recited in U.S. Pat. No. 5,366,138 (Vela et al.), includes 30-32% TiC, 9-10% Cr, 3-6.5% Co, 3-4.5% Ni, 2-4% Mo, 0-1% Al, 1-2% Ti, 0-1% Cr, and 40-50% Fe. The ultra-hard, rounded titanium carbide grains are uniformly distributed throughout a hardenable steel alloy matrix. Fabrication in the annealed state is accomplished with ordinary tools and equipment, followed by conventional heat treatment to obtain maximum hardness.
WC—Co based cemented carbides include a range of composite materials which contain hard carbide particles bonded together by a metallic binder. The proportion of carbide phase is generally between 70-97% of the total weight of the composite and its grain size averages between 0.2 and 14 μm. For example, a typical cobalt bound tungsten carbide material is disclosed in U.S. Pat. No. 4,923,512 (Timm et al.). Timm et al. recites a composition having WC in an amount of 83 to 99 weight % and cobalt in an amount of 1-18 weight %. Tungsten carbide (WC), the hard phase, together with cobalt (Co), the binder phase, forms the basic cemented carbide structure. In addition to WC—Co compositions, cemented carbide may contain small proportions of secondary carbides such as titanium carbide (TiC), tantalum carbide (TaC), and niobium carbide (NbC). These secondary carbides are mutually soluble and can also dissolve a high proportion of tungsten carbide. In addition, cemented carbides are produced which have the cobalt binder phase alloyed with, or completely replaced by, other metals such as nickel (Ni), chromium (Cr), molybdenum (Mo), iron (Fe) or alloys of these elements. Thus, there are typically three individual phases which make up a cemented carbide, the α-phase of tungsten carbide, the β-phase of a binder material (e.g. Co, Ni, etc.), and the γ-phase which is a single or solid solution carbide phase (e.g., of WC and TiC, and/or TaC, and/or NbC, and/or nitrides or carbonitrides).
Ferro-TiC alloys, although generally effective in wear-resistance applications, are more expensive than comparable WC—Co alloys and are more difficult to work. For example, while WC—Co alloys can be inexpensively and easily silver soldered or brazed in air to a die body, Ferro-TiC alloys cannot be silver soldered or brazed directly to the die by conventional methods.
WC—Co materials, though having similar corrosion and wear resistance to Ferro-TiC alloys when used as orifice nibs, suffer from undesirably high thermal conductivity. High thermal conductivity leads to the freezing of the polymer in the die orifice as the orifice nib conducts heat away from the polymer being extruded, due to the cooling effect of circulating water at the surface. This is predominant when filler materials are involved, such as in polypropelene.
U.S. Pat. No. 6,521,353 to Majagi et al., which is incorporated herein by reference, discloses a hard metal comprising a major amount of tungsten carbide and a minor amount of titanium carbide which are cemented together with a binder material of cobalt and nickel.
There continues to be a need for hard metal alloy materials for use in pelletizing die faces and other high-wear applications that have ultra-low thermal conductivity comparable to ceramic materials, high wear and corrosion resistance, and are relatively inexpensive, easy to manufacture and easier to join with steel in comparison with ceramic materials.